I was thinking more like this. Every venue you play is should have it, and it will do the trick most of the time as long as you keep your bass in the right spot.
OP, as a heads-up, unless you already have an XLR to USB adapter (or other audio equipment), you can't get the SM57 to work with your computer for $100. You'd need to buy at least this which I wouldn't recommend anyway but that isn't the point.
If you're willing to monitor product availability, I would order it from Sweetwater and use your GC for a mic. It's only $40 at Sweetwater.
It's complete for me because it has MIDI in/out, RCA out, and it charges your ipad while you use it. It's INCOMPLETE for a lot of people because it doesn't have XLR in. I overcome this by using a mixer in front of the StudioConnect.
Soooo... in a perfect world, you could construct the following chain:
That drains your funds without getting you any cables. You can aim a little lower and get a SM48 ($30 at Amazon) and have enough room to cover cables and taxes.
To get a good lay of the land in terms of interfaces, I'd shop at Sweetwater and buy at Amazon. It's easier to get a grip on what's out there. http://www.sweetwater.com/c1058--iPad_iPhone_Interfaces/low2high
This guy with the XLR-to-USB adapter (This allows you to connect the mic directly to your computer without needing to buy a full blown amplifier, which you don't need unless you need more control over the output). This mic and the SM58 are both great for vocals, but from what I read before ordering the SM57 is that it's better for all around recording, and I don't plan on recording just my voice all the time.
On the other hand, it does appear to be really good at doing that. :D
What's your budget? This is a great and standard inexpensive microphone, I recently got one and am very happy with it.